1. Field of Invention
This invention relates to a technique which is suitably adapted for production of a substrate for a liquid crystal device, and a liquid crystal device and projection type display device based on the use thereof This invention relates more particularly to a light shielding structure of the substrate for the liquid crystal device which is used as a pixel switching element of a thin film transistor (to be abbreviated as TFT hereinafter).
2. Description of Related Art
Conventionally, a liquid crystal device is put into practice where pixel electrodes have been arranged in the form of a matrix on a glass substrate, and TFTs made of an amorphous silicon film or a polysilicon film have been prepared in correspondence with each pixel electrode, and which is so constructed as to drive a liquid crystal by applying a voltage through the TFT to each pixel electrode.
Among such liquid crystal devices, one incorporating a polysilicon film of which it is possible to assemble peripheral driving circuits such as a shift register or the like on the same substrate through the same process, allows a high density integration of circuit elements and attracts general attention.
With the liquid crystal device incorporating TFTs, the top of a TFT for driving a pixel electrode (to be referred to as a pixel TFT hereinafter) is covered by a light shielding film such as a chromium film called a black matrix (or a black stripe), which is placed on the opposite substrate. This is to prevent the channel region of the TFT from being exposed to direct light which would otherwise cause a leakage current. However, a leakage current caused by exposure of the TFT to stray light may arise as a result of light reflected from a polarizer placed on the back surface of the liquid crystal device, not to mention the adverse effects due to incident light itself.
To minimize such leakage current due to reflective light, an invention is proposed in which the back surface of the TFT is also covered by a light shielding film (Japanese Patent Publication, No. Hei 3-52611). If the light shielding film is placed on the back surface of the TFT such that it exceeds in size the opening of the black matrix placed on the opposite substrate, incident light strikes directly on the light shielding film, and light reflected therefrom illuminates the channel region of the TFT, which may cause it to generate a leakage current. This is because, when a process necessary for the placement of a light shielding film on the back surface of the TFT is put into practice, precise alignment of a black matrix placed on the opposite matrix with a pixel region placed on the substrate for the liquid crystal device is difficult, and thus incident light through the opposite substrate directly impinges and is reflected on the part of light shielding film that exceeds in size the opening of the black matrix. As a result, the channel region of TFT is illuminated, causing the leakage current to flow. Particularly when alignment of the light shielding layer placed on the substrate for the liquid crystal device with the black matrix takes place with a large error, light reflected from the surface of light shielding film increases considerably, and, as the channel region is illumined by this reflective light, a leakage current from the TFT is increased, resulting in a degraded display as a result of flaws such as cross talks or the like.
The object of this invention is to provide a technique which, when applied to a liquid crystal device, can minimize a leakage current generated from a TFT exposed to light. Another object of this invention is to provide a technique which can minimize a leakage current from a TFT exposed to light, without resorting to a black matrix placed on the opposite substrate.
To achieve the above objects, this invention is characterized by providing a substrate for a liquid crystal device, comprising:
a plurality of data lines formed on the substrate;
a plurality of scan lines crossing the plurality of data lines;
a plurality of thin film transistors connected to the plurality of data lines and scan lines; and
a plurality of pixel electrodes connected to the plurality of thin film transistors; wherein:
a first light shielding film formed at least below a channel region of the thin film transistor, and the junctions between the channel region and source/drain regions; and
a second light shielding film formed above the channel region and the junctions between the channel region and the source/drain regions.
According to the substrate for a liquid crystal device as described above, light impinging from above on the channel region and on the junctions between the channel region and the source/drain regions is shielded by the first shielding film, and light impinging from below is blocked by the second light shielding film. Through this arrangement, a leakage current which would otherwise be generated in the TFT exposed to light can be stably reduced.
The substrate for the liquid crystal device in another aspect is characterized in that the first light shielding film may be a metal film selected from the group consisting of a tungsten film, titanium film, chromium film, tantalum film and molybdenum film, or an alloy film thereof.
According to the substrate for the liquid crystal device as described above, when a metal film or a metal alloy film which is highly impenetrable to light and highly electrically conductive is used as a first light shielding film, it effectively acts as a light shielding film against reflective light from the back surface of the substrate for the liquid crystal device, and protects the channel region and the junctions between the channel region and the source/drain regions from exposure to light.
The substrate for the liquid crystal device in another aspect is characterized in that a first lead extending from the first light shielding film is electrically connected to a constant potential line outside a pixel display region.
According to the substrate for the liquid crystal device as described above, when the first light shielding film is formed in a floating state below the channel region of the TFT, irregular potential differences are generated between different terminals of the TFT, which may affect the TFT""s performance. As a measure against such inconvenience, the first light shielding film must be stabilized at a specific potential level. This is the reason why the first lead extending from the first light shielding film is connected to a line having a constant potential such as a ground potential, outside a display region. This measure serves for inhibiting generation of potential differences among different terminals of the TFT, thus preventing alteration of TFT performance and occurrence of degraded display quality.
The substrate for the liquid crystal device in another aspect is characterized in that the first lead extending from the first light shielding film is formed along and beneath the scan line.
According to the substrate for the liquid crystal device as described above, the first lead extending from the first light shielding film is formed along and below the scan line. Through this arrangement it is possible for the lead to run without encroaching the aperture of the pixel. However, the first light shielding film is placed below the scan line and is positioned with respect to the side of scan line close to the aperture area of the pixel in such a way as to prohibit the direct impingement of incident light on the surface of first light shielding film.
The substrate for the liquid crystal device in another aspect is characterized in that a width of the first lead extending from the first light shielding film is less than a width of the scan line formed above it.
The substrate for the liquid crystal device in another aspect is characterized in that the first lead extending from the first light shielding film is covered by the scan line formed above it.
According to the substrate for the liquid crystal device as described above, the scan line can prevent the first lead extending from the first light shielding film from being directly exposed to incident light and thus from reflecting incident light.
The substrate for the liquid crystal device in another aspect is characterized in that a capacitance line which is formed on the same layer as that of the scan line to add a capacitance to the pixel is placed in parallel with the scan line, and has below it a second lead extending from the first light shielding film.
According to the substrate for the liquid crystal device as described above, the second line extending from the first light shielding film, by being placed below the capacitance line which runs parallel with the scan line, and an added capacitor is formed by the second line, the drain region of the TFT and a first interlevel insulating film as a dielectric material. Through this arrangement it is possible to increase an extra capacitance without reducing the aperture of the pixel.
The substrate for the liquid crystal device in another aspect is characterized in that a third lead extending from the first light shielding film is placed along and below a data line.
According to the substrate for the liquid crystal device as described above, the third lead extending from the first light shielding film may be formed along and below the data line. This lead extension, however, should be arranged such that the first light shielding film placed below the data line is covered by the data line at the areas where the data line comes into contact with or comes very close to the pixel aperture region, in order to prevent the surface of the first light shielding film from being directly exposed to incident light.
The substrate for the liquid crystal device in another aspect is characterized in that the data line also acts as a second light shielding film, and is made of any metal film selected from an aluminum film, tungsten film, titanium film, chromium film, tantalum film and molybdenum film, or an alloy film thereof.
According to the substrate for the liquid crystal device as described above, preparing the data line from a metal film or a metal alloy film makes it possible for the data line to also act as a second light shielding film. This arrangement makes it unnecessary to prepare a layer only for light shielding.
The substrate for the liquid crystal device in another aspect is characterized in that the third lead extending from the first light shielding film has a smaller width than that of data line.
The substrate for the liquid crystal device in another aspect is characterized in that the channel region and the junction between the channel region forms and the source/drain region are placed beneath the data line, and that the first light shielding film placed beneath the channel region and the junction between the channel region and the source/drain region is covered by the data line at least on the part underlying the channel region and the junction between the channel region forms and the source/drain region.
According to the substrate for the liquid crystal device as described above, at least the channel region and the junctions between the channel region and the source/drain regions are shielded by the data line (second light shielding film) from exposure to incident light from above. When incident light comes from above, it is necessary to protect the channel region and the junctions between the channel region and the source/drain regions from exposure to light reflected from the surface of the first light shielding film. To achieve this, the data line is formed in such a way as to totally cover the first light shielding film placed beneath the channel region and the junctions between the channel region and the source/drain regions.
The substrate for the liquid crystal device in another aspect is characterized in that LDD regions are formed at the junctions between the channel region and the source/drain regions.
According to the substrate for the liquid crystal device as described above, the junctions of the channel region with source/drain regions of a pixel TFT are prepared as LDD regions, which enables the reduction of a leakage current which would otherwise result when the TFT is turned off. However, when the LDD region is exposed to light, generally electrons within are readily excited. Thus, it is necessary to cover the LDD region with the first and second light shielding films from above and below, as is the case with the channel region.
The substrate for the liquid crystal device in another aspect is characterized in that the junctions between the channel region and the source/drain regions are formed as offset regions.
According to the substrate for the liquid crystal device as described above, the junctions between the channel region of the pixel TFT and the source/drain regions are formed as offset regions not doped with impurity ions, which enables the reduction of a leakage current which would otherwise result when the TFT is turned off. However, when the offset region is exposed to light, generally electrons within are readily excited as in the LDD region. Therefore, like the channel region, the offset regions are so formed as to be totally covered by the first and second light shielding films from above and below.
The substrate for the liquid crystal device in another aspect is characterized in that the scan line is made of any metal film selected from a tungsten film, titanium film, chromium film, tantalum film and molybdenum film, or of a metal alloy film thereof.
According to the substrate for the liquid crystal device as described above, the scan line is made at least of a metal film or a metal alloy film which makes it possible for the scan line to also act as a light shielding film. Because through this arrangement it is possible for the scan line as well as the data line to act as a light shielding film, placement of a black matrix on the opposite substrate can be safely omitted, by forming all the sides surrounding the pixel electrode so as to overlap with the data lines and the scan lines.
The substrate for the liquid crystal device in another aspect is characterized in that the smallest distance L1 from the lateral edges of first light shielding film to the channel region is made 0.2 xcexcmxe2x89xa6L1xe2x89xa64 xcexcm.
According to the substrate for the liquid crystal device as described above, it is possible to prevent adverse effects due to reflective light from the first light shielding film.
The substrate for the liquid crystal device in another aspect is characterized in that the smallest distance L2 from the lateral edges of the second light shielding film to the lateral edges of first light shielding film is made 0.2 xcexcmxe2x89xa6L2.
According to the substrate for the liquid crystal device as described above, it is possible to prevent adverse effects due to reflective light from the first light shielding film.
The substrate for the liquid crystal device in another aspect is characterized in that the substrate for the liquid crystal device and an opposite substrate with an opposite electrode are placed with a specified interval in between, and that liquid crystal is inserted into the space between the substrate for the liquid crystal device and the opposite substrate.
According to the substrate for the liquid crystal device as described above, the substrate for the liquid crystal device and the opposite substrate are bonded together by a specified cell gap, liquid crystal is injected into the space between the substrate for the liquid crystal device and the opposite substrate, and a voltage is applied across the liquid crystal to achieve a gray scale. This liquid crystal device, as long as it receives incident light only through the opposite substrate, ensures a high grade display of images free from adverse effects due to stray light.
The liquid crystal device in another aspect is characterized in that a third light shielding film is formed on the opposite substrate.
According to the liquid crystal device as described above, on the opposite substrate is formed a black matrix (third light shielding film) with a high light shielding property which is made of a metal film such as chromium film or a black matrix composed of an organic substance. The pixel TFT placed on the substrate for the liquid crystal device is prevented by the black matrix from being directly exposed to light. This arrangement makes it possible to provide a liquid crystal device with a display capable of reproducing high quality images.
The liquid crystal device in another aspect is characterized in that the third light shielding film covers at least the first light shielding film.
According to the liquid crystal device as described above, the first light shielding film placed on the substrate for the liquid crystal device is covered by the black matrix (third light shielding film) on the opposite substrate, which makes it possible for the first light shielding film to be shielded from direct exposure to incident light. This arrangement prevents light reflected from the surface of light shielding film from impinging on the channel region of the TFT and the junctions between the channel region and the source/drain regions, which enables the reduction of a leakage current which would otherwise arise if the TFT were exposed to light.
The liquid crystal device in another aspect is characterized in that small lenses are arranged in the form of a matrix on the opposite electrode in correspondence with the plurality of pixel electrodes placed on the substrate for the liquid crystal display device.
According to the liquid crystal device as described above, the small lens mounted on the opposite electrode converges light onto the pixel aperture region on the substrate for the liquid crystal device. The first light shielding film is placed on the substrate for the liquid crystal device such that light converged by the small lens, even when reflected from the back surface of the substrate for the liquid crystal device, is prevented from impinging on the channel region of the pixel TFT. Accordingly, even when light is converged by the small lens into a strong flux, it does not affect the TFT performance, and thus production of a liquid crystal device capable of reproducing bright, high quality images will be ensured.
The projection type display system in another aspect is characterized by comprising a light source, a liquid crystal device to transmit or reflect light from the light source, after having modulated it, and an optical projection means which receives the modulated light sent from the liquid crystal device, and converges and enlarges it through projection.
According to the projection type display system as described above, the projection type display system has a liquid crystal device of this invention, and can prevent the entry of stray light through the first light shielding film on the substrate for the liquid crystal device, even when the back surface of the substrate for the liquid crystal device is exposed to such light as reflected from a dichroic prism or the like. Accordingly, even when light is intensified and such intensified light is incident on the liquid crystal device, it does not affect the TFT performance, and thus production of a projection type display system capable of reproducing bright, high quality images will be ensured.
Operation and other advantages of this invention will be clearly described with reference to preferred embodiments given below.